Electron microscopic observations of muscle dedifferentiation in regenerating Amblystoma limbs☆

[摘要] RegeneratinglimbsofAmblystomapunctatumlarvaewerefixedinosmicacid,sectioned,andstudiedwiththeelectronmicroscope.Thepresentreportdescribesthechangeswhichoccurinthesyncytialmusclefibersofthestumpastheytransformintoundifferentiated,mononucleateblastemacellsduringthe2–6-dayperiodafteramputation.Onlythenucleatedcellunitsderivedfromthedissociatingmusclefibercompletetheprocessofdedifferentiation.Anucleatefragmentsapparentlyarelysed.Nucleibecomeroundedinshape,thechromatinmorediffuselygranular,andnucleolimoreprominent.Thenucleocytoplasmicratioisincreasedbylossofcytoplasmbyfragmentationduringdedifferentiation.Boththeintracellularandextracellularproductsofcelldifferentiationarelostduringdedifferentiation.MyofibrilsfragmentalongtheAandIbandsandthendisintegrate.TheZsubstancebehavesasadistinctextrafilamentouscomponentduringdisorganizationofthemyofibril,anditsbehavioriscompatiblewiththeconceptthatitisanadhesivematerialservingnormallytobindsarcomerestogether.Individualmyofibrils,undetectedheretofore,persistinthecytoplasmofsomeofthemononucleatecellsforashortperiodaftertheirseparationfromthemusclefiber.Thusthereremainsnodoubtthatmononucleatecellsinthevicinityofintactstumpmuscledoarisefrommultinucleatemusclefibers.Bythetimethetransformationtoblastemacelliscompleted,allthemyofibrilshavedisappearedfromthecytoplasm.Thenewplasmamembranesseparatingmusclefragmentsappeartobeformedbyfusionofsmallvesicleswhichareprobablyderivedlargelyfromtheendoplasmicreticulum.Theendoplasmicreticulum,whichishighlydevelopedindifferentiatedmuscle,breaksupintosmallvesiclesindedifferentiatingmuscle.TheGolgiapparatusalsobecomesunorganizedinappearance,whereasthemitochondriashowrelativelylittlechange.Oneofthemoststrikingchangesinthecytoplasmisitsacquisitionofnumerousfreeribonucleoproteingranules,afeaturecharacteristicofundifferentiatedcellsingeneral.Thesefeaturestogetherwiththenuclearchangesdescribedaboveindicatethatthededifferentiatingmusclecellisprobablybeginningactivecytoplasmicproteinsynthesis.Theydispeltheideathatthemusclemightbedegenerating.Theendresultoftheprocessofmusclededifferentiationisanundifferentiated-appearingcellwhichcannotbedistinguishedfromtheblastemacellsderivedfromcartilageandothertissuesofthestump.Byvirtueofitsacquisitionofsuchembryonicfeaturesasfreecytoplasmicribonucleoproteingranulesandprominentnucleoli,aswellaslossofdifferentiatedstructure,thededifferentiatedblastemacellseemstoberemarkablywellequippedfortheensuingrapidproliferationofalargecellularmasscapableofredifferentiationintoacompletelimb.

[学科分类] 生物科学（综合）